Crude oil is the world's main source of hydrocarbons that are used as fuel and petrochemical feedstock. One overriding problem in exploring for hydrocarbons in the subsurface is related to the probing and characterization of an environment that cannot be seen. Similarly, after a hydrocarbon deposit has been discovered and is ready to be developed and exploited, many assumptions must be made by reservoir geologists and reservoir engineers in the modeling of a large volume of rock which cannot be seen.
Subsurface reservoir data is traditionally acquired by lowering probes into boreholes for purposes of sampling and/or probing the reservoir and from images obtained through seismography. In the first instance, the data is handicapped by the insufficiency of the resulting data, by virtue of being sourced from a single 6-inch hole, thus giving a very narrow of a view of the reservoir as a whole. Interpreted seismic volumes, on the other hand, typically give too broad of a view due to their imaging quality and resolution inadequacies. Even combining the two data types, does not enable for the mapping of exact high permeability pathways.
The integration of available geological, geophysical, petrophysical engineering, and drilling data makes interesting inroads into the detection, mapping and predictive modeling of high permeability pathways. The final uncertainty of integrated models, however, can only be marginally better than the average uncertainty inherent in each the various methods used. Mix and integrate the data as much as one may, the broad brush strokes on reservoir map deliverables will remain just that: broad brush. For example, a 0.5 mm scribble drawn on a 1:200,000 scale map to represent a fracture in the subsurface, is akin to depicting a fracture with a 200 meter aperture because of the width of the scribble relative to the scale of the map. Nor does the scribble reveal the precise path that the fluids are likely to take.
Additionally, as oil fields mature, it is expected that fluid injection for pressure support (i.e., secondary enhanced oil recovery) will increasingly tend to erratically invade and irregularly sweep the residual oil leg. Concerns have led to needs to identify, detect and map pathways that may lead injected fluids prematurely updip along encroachment fingers. More often than not, the encroachment materializes faster than even the worst case expectations, and commonly in quite unpredictable directions. Moreover, premature encroachment is commonly tortuous and will change direction in 3D volume. This type of tortuousity renders high permeability pathway prediction nearly impossible to satisfactorily pin down. In spite of an arsenal of cutting-edge technologies that can be thrown at such problems, high permeability pathway prediction capability continues to suffer from high levels of uncertainty.
Even with current technology, it is impossible to work out and predict an exact pathway that fluid fingering will take as it invades deep into an oil leg, much less where it will go next. Engineering data (e.g. water arrival data, including, water arrival detected in an oil producing well, flowmeter data, test pH build-up, pressure data, and production/injection data), although mostly acquired at the borehole, are typically correlated aerially. The resultant maps provide an indirect, unreliable and a crude way of trying to depict the geology of a reservoir. The resultant maps are interpretive, and reservoir engineers are the first to dissociate them from being accurate reflections of specific geologic features. Moreover, the map resolutions are too broad to even remotely represent most geological features that would commonly be associated with high permeability pathways.
Other interwell methods to map permeability pathways are, likewise, handicapped by resolution problems. Geophysical technologies rooted in interpreting 3D, 4D, shear wave, or multi-component volumes; even when utilizing ever-developing clarity and resolution enhancing software packages, still only render a generalized mapping of a miniscule sampling of some faults in the general area where they may or may not be located.
In carbonate rock formations, fractures having apertures that are measured in millimeters, or geobodies only centimeters across, can provide the necessary plumbing to take injected fluid past matrixed oil. To further illustrate this, a 3 cm wide fracture with no displacement may, under pressure, move fluids at several Darcies. These dimensions cannot be seen by current interpretive geophysical devices. Subsequently, the fault lines drawn on reservoir structure maps cannot be considered more than broad arrows pointing out a general direction; and not a depiction of actual permeability pathways. Furthermore, geophysically-interpreted data must be augmented by a solid understanding of the regional stress-strain regimes in order to filter out fracture swarms which may not be contributing to premature fluid breakthroughs.
Dyes and radioactive chemical tracers that can be introduced with injected fluids can be helpful locally, but generally do not reveal the actual pathway that is taken by the host fluid from the entry well to the detection well. Borehole detection methods are the most exact, but are also plagued with major shortcomings, such as for mapping purposes, wellsite data must be extrapolated and transformed into interwell information: Extrapolation in itself creates many problems. Some disadvantages associated with molecular (chemical) tracers include diffusivity and adsorption. Molecular tracers, due to their small size, tend to diffuse into all of the small pores of a matrix (as compared with larger tracers), and thus take longer periods of time to travel between the injection well and the production well. Additionally, adsorption of the molecular tracers can also be a factor, requiring the injection of much larger quantities of the chemical tracers than is desired.
The slightest shifts in water depth, measured in decimeters, can create worlds of difference in sediment deposition. Rock minerals, especially carbonates, are in continuous “life long” effective diagenesis from the instant of deposition. Carbonate porosity is dictated by deposition and unceasingly altered by diagenesis.
Geostatistical distribution of attributes, including fractures detected on borehole image logs, at the wellbore, is only statistical, and natural geological landscapes are too variable to respond comfortably to the smooth, clean logic of mathematics. There are no two features in carbonate rocks that are the same.
Nanotechnology brings new and different capabilities into upstream exploration and production. The industry desires strong, stable, friction resistant, and corrosion combatant materials in virtually all of its operations and processes (e.g., pipes, casings, drill strings, production tubings, and drill bits). These requirements are more favorably addressed with a bottom-up approach for material design and fabrication, and by employing nanofabricated particles for use in drilling, completion, stimulation, and injection fluids. Use of these materials allows faster drilling, prevents near wellbore damage, mine hydraulic fractures, plug water thief zones, reduce waterflood fingering, encourage or enhance oil production, and prevent water breakthroughs. It is hoped that the use of certain nano-based agents may soon lead to the development and deployment of sensing and intervention devices that can help delineate the waterflood front, identify bypassed oil, and map super permeability zones in-situ in the underground. The capabilities become limitless with the possibility of having functionalized molecular agents that “illuminate” the reservoir and possibly intervene to rectify adverse transport conditions in the medium.
As worldwide petroleum reserves decrease and their recovery becomes increasingly difficult, methods for locating and mapping petroleum reservoirs becomes more and more critical. Due to the high pressures and temperatures that are encountered in subsurface formations, materials that are able to withstand these conditions are needed. Thus, there is a need for the development of new materials for use with the mapping of petroleum reservoirs.